Light microscopic observations of the contacts made between two spindle afferent types and alpha-motoneurons in the cat trigeminal motor nucleus

Molecular, Morphological and Electrophysiological Differences between Alpha and Gamma Motoneurons with Special Reference to the Trigeminal Motor Nucleus of Rat

The muscle contraction during voluntary movement is controlled by activities of alpha- and gamma-motoneurons (αMNs and γMNs, respectively). In spite of the recent advances in research on molecular markers that can distinguish between αMNs and γMNs, electrophysiological membrane properties and firing patterns of γMNs have remained unknown, while those of αMNs have been clarified in detail. Because of the larger size of αMNs compared to γMNs, blindly or even visually recorded MNs were mostly αMNs, as demonstrated with molecular markers recently. Subsequently, the research on αMNs has made great progress in classifying their subtypes based on the molecular markers and electrophysiological membrane properties, whereas only a few studies demonstrated the electrophysiological membrane properties of γMNs. In this review article, we provide an overview of the recent advances in research on the classification of αMNs and γMNs based on molecular markers and electrophysiological membrane properties, and discuss their functional implication and significance in motor control.

The ultrastructure of macaque mesencephalic trigeminal nucleus neurons

Primary afferents originating from the mesencephalic trigeminal nucleus provide the main source of proprioceptive information guiding mastication, and thus represent an important component of this critical function. Unlike those of other primary afferents, their cell bodies lie within the central nervous system. It is believed that this unusual central location allows them to be regulated by synaptic input. In this study, we explored the ultrastructure of macaque mesencephalic trigeminal nucleus neurons to determine the presence and nature of this synaptic input in a primate. We first confirmed the location of macaque mesencephalic trigeminal neurons by retrograde labeling from the masticatory muscles. Since the labeled neurons were by far the largest cells located at the edge of the periaqueductal gray, we could undertake sampling for electron microscopy based on soma size. Ultrastructurally, mesencephalic trigeminal neurons had very large somata with euchromatic nuclei that sometimes displayed deeply indented nuclear membranes. Terminal profiles with varied vesicle characteristics and synaptic density thicknesses were found in contact with either their somatic plasma membranes or somatic spines. However, in contradistinction to other, much smaller, somata in the region, the plasma membranes of the mesencephalic trigeminal somata had only a few synaptic contacts. They did extend numerous somatic spines of various lengths into the neuropil, but most of these also lacked synaptic contact. The observed ultrastructural organization indicates that macaque trigeminal mesencephalic neurons do receive synaptic contacts, but despite their central location, they only avail themselves of very limited input.

Subcellular Localization of Homomeric TASK3 Channels and Its Presumed Functional Significances in Trigeminal Motoneurons

Somatic expressions of either heteromeric TASK1/3 or homomeric TASK1/1 channels have been reported in various neurons, while expression of homomeric TASK3/3 channels has been re-ported only in dendrites. However, it is not known why homomeric TASK3/3 channels are hardly seen in somata of CNS neurons. Given the absence of somatic TASK3/3 channels, it should be clarified why dendritic expression of TASK3/3 channels is inevitable and necessary and how differentially distributed TASK1/1 and TASK3/3 channels play roles in soma-to-dendritic integration. Here, we addressed these questions. We found that TASK3-transfected HEK293 cells showed decreases in cell volume after being transferred from the cultured medium to HEPES Ringer, suggesting that expressions of TASK3 channels in cell bodies cause an osmolarity problem. Using TASK1- and TASK3-transfected oocytes, we also found that cGMP application slightly suppressed TASK3 currents while it largely enhanced TASK1 currents, alleviating the difference between TASK1 and TASK3 currents at physiological pH. As larger motoneurons have extensive dendritic trees while smaller motoneurons have poor ones, cGMP could integrate Ia-EPSPs to recruit small and large motoneurons synchronously by differentially modulating TASKI and TASK3 channels which were complementary distributed in soma and dendrites of motoneurons in the dorsolateral part of the trigeminal motor nucleus.

Synaptic connectivity of urinary bladder afferents in the rat superficial dorsal horn and spinal parasympathetic nucleus

That visceral sensory afferents are functionally distinct from their somatic analogues has been known for a long time but the detailed knowledge of their synaptic connections and neurotransmitters at the first relay nucleus in the spinal cord has been limited. To provide information on these topics, we investigated the synapses and neurotransmitters of identified afferents from the urinary bladder to the superficial laminae of the rat spinal dorsal horn (DH) and the spinal parasympathetic nucleus (SPN) by tracing with horseradish peroxidase, quantitative electron microscopical analysis, and immunogold staining for GABA and glycine. In the DH, most bladder afferent boutons formed synapses with 1-2 postsynaptic dendrites, whereas in the SPN, close to a half of them formed synapses with 3-8 postsynaptic dendrites. The number of postsynaptic dendrites and dendritic spines per bladder afferent bouton, both measures of synaptic divergence and of potential for synaptic plasticity at a single bouton level, were significantly higher in the SPN than in the DH. Bladder afferent boutons frequently received inhibitory axoaxonic synapses from presynaptic endings in the DH but rarely in the SPN. The presynaptic endings were GABA- and/or glycine-immunopositive. The bouton volume, mitochondrial volume, and active zone area, all determinants of synaptic strength, of the bladder afferent boutons were positively correlated with the number of postsynaptic dendrites. These findings suggest that visceral sensory information conveyed via the urinary bladder afferents is processed differently in the DH than in the SPN, and differently from the way somatosensory information is processed in the spinal cord.

Projection and synaptic connectivity of trigeminal mesencephalic nucleus neurons controlling jaw reflexes

Neurons in the trigeminal mesencephalic nucleus (Vmes) receive deep sensation (proprioception) from jaw-closing muscle spindles and periodontal ligaments and project primarily to the jaw-closing motoneuron pool (jaw-closing nucleus) of the trigeminal motor nucleus and to the supratrigeminal nucleus. Numerous articles have described the morphology and physiology of the central projections of Vmes afferents originating from the muscle spindles and periodontal ligaments. However, no report has provided a detailed description of projection and synaptic connectivity, especially of single afferents, and their functional implications. In this review, we reanalyze data obtained by single intra-axonal recording and labeling of functionally identified Vmes muscle spindle afferents and periodontal ligament afferents and by electron microscopic observation of their projection features and synaptic organization of boutons, to compare the data for the jaw-closing nucleus and supratrigeminal nucleus. Our analysis shows that each Vmes afferent type has characteristic projection pattern and synaptic feature that may be important in jaw-reflex control.

Extrasynaptic homomeric glycine receptors in neurons of the rat trigeminal mesencephalic nucleus

The neurons in the trigeminal mesencephalic nucleus (Vmes) innervate jaw-closing muscle spindles and periodontal ligaments, and play a crucial role in the regulation of jaw movements. Recently, it was shown that many boutons that form synapses on them are immunopositive for glycine (Gly+), suggesting that these neurons receive glycinergic input. Information about the glycine receptors that mediate this input is needed to help understand the role of glycine in controlling Vmes neuron excitability. For this, we investigated the expression of glycine receptor subunit alpha 3 (GlyRα3) and gephyrin in neurons in Vmes and the trigeminal motor nucleus (Vmo), and the Gly+ boutons that contact them by light- and electron-microscopic immunocytochemistry and quantitative ultrastructural analysis. The somata of the Vmes neurons were immunostained for GlyRα3, but not gephyrin, indicating expression of homomeric GlyR. The immunostaining for GlyRα3 was localized away from the synapses in the Vmes neuron somata, in contrast to the Vmo neurons, where the staining for GlyRα3 and gephyrin were localized at the subsynaptic zones in somata and dendrites. Additionally, the ultrastructural determinants of synaptic strength, bouton volume, mitochondrial volume, and active zone area, were significantly smaller in Gly+ boutons on the Vmes neurons than in those on the Vmo neurons. These findings support the notion that the Vmes neurons receive glycinergic input via putative extrasynaptic homomeric glycine receptors, likely mediating a slow, tonic modulation of the Vmes neuron excitability.

Electrophysiological and Morphological Properties of α and γ Motoneurons in the Rat Trigeminal Motor Nucleus

The muscle contraction during voluntary movement is regulated by activities of α- and γ-motoneurons (αMNs and γMNs, respectively). The tension of jaw-closing muscles can be finely tuned over a wide range. This excellent function is likely to be achieved by the specific populations of αMNs innervating jaw-closing muscles. Indeed, we have recently demonstrated that in the rat dorsolateral trigeminal motor nucleus (dl-TMN), the size distribution of αMNs was bimodal and the population of smaller αMNs showed a size distribution similar to that of γMNs, by immunohistochemically identifying αMNs and γMNs based on the expressions of estrogen-related receptor gamma (Err3) and neuronal DNA binding protein NeuN together with ChAT. This finding suggests the presence of αMNs as small as γMNs. However, differences in the electrophysiological membrane properties between αMNs and γMNs remain unknown also in the dl-TMN. Therefore, in the present study, we studied the electrophysiological membrane properties of MNs in the dl-TMN of infant rats at postnatal days 7–12 together with their morphological properties using whole-cell current-clamp recordings followed by immunohistochemical staining with an anti-NeuN and anti-ChAT antibodies. We found that the ChAT-positive and NeuN-positive αMNs were divided into two subclasses: the first one had a larger cell body and displayed a 4-aminopyridine (4-AP)-sensitive current while the second one had a smaller cell body and displayed a less prominent 4-AP-sensitive current and a low-threshold spike, suitable for their orderly recruitment. We finally found that γMNs showing ChAT-positive and NeuN-negative immunoreactivities had smaller cell bodies and displayed an afterdepolarization mediated by flufenamate-sensitive cation current. It is suggested that these electrophysiological and morphological features of MNs in the dl-TMN are well correlated with the precise control of occlusion.

The Possible Role of TASK Channels in Rank-Ordered Recruitment of Motoneurons in the Dorsolateral Part of the Trigeminal Motor Nucleus

Because a rank-ordered recruitment of motor units occurs during isometric contraction of jaw-closing muscles, jaw-closing motoneurons (MNs) may be recruited in a manner dependent on their soma sizes or input resistances (IRs). In the dorsolateral part of the trigeminal motor nucleus (dl-TMN) in rats, MNs abundantly express TWIK (two-pore domain weak inwardly rectifying K channel)-related acid-sensitive-K(+) channel (TASK)-1 and TASK3 channels, which determine the IR and resting membrane potential. Here we examined how TASK channels are involved in IR-dependent activation/recruitment of MNs in the rat dl-TMN by using multiple methods. The real-time PCR study revealed that single large MNs (>35 μm) expressed TASK1 and TASK3 mRNAs more abundantly compared with single small MNs (15-20 μm). The immunohistochemistry revealed that TASK1 and TASK3 channels were complementarily distributed in somata and dendrites of MNs, respectively. The density of TASK1 channels seemed to increase with a decrease in soma diameter while there were inverse relationships between the soma size of MNs and IR, resting membrane potential, or spike threshold. Dual whole-cell recordings obtained from smaller and larger MNs revealed that the recruitment of MNs depends on their IRs in response to repetitive stimulation of the presumed Ia afferents. 8-Bromoguanosine-cGMP decreased IRs in small MNs, while it hardly changed those in large MNs, and subsequently decreased the difference in spike-onset latency between the smaller and larger MNs, causing a synchronous activation of MNs. These results suggest that TASK channels play critical roles in rank-ordered recruitment of MNs in the dl-TMN.

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Central connectivity of transient receptor potential melastatin 8-expressing axons in the brain stem and spinal dorsal horn

Transient receptor potential melastatin 8 (TRPM8) ion channels mediate the detection of noxious and innocuous cold and are expressed by primary sensory neurons, but little is known about the processing of the TRPM8-mediated cold information within the trigeminal sensory nuclei (TSN) and the spinal dorsal horn (DH). To address this issue, we characterized TRPM8-positive (+) neurons in the trigeminal ganglion and investigated the distribution of TRPM8+ axons and terminals, and their synaptic organization in the TSN and in the DH using light and electron microscopic immunohistochemistry in transgenic mice expressing a genetically encoded axonal tracer in TRPM8+ neurons. TRPM8 was expressed in a fraction of small myelinated primary afferent fibers (23.7%) and unmyelinated fibers (76.3%), suggesting that TRPM8-mediated cold is conveyed via C and Aδ afferents. TRPM8+ axons were observed in all TSN, but at different densities in the dorsal and ventral areas of the rostral TSN, which dominantly receive sensory afferents from intra- and peri-oral structures and from the face, respectively. While synaptic boutons arising from Aδ and non-peptidergic C afferents usually receive many axoaxonic contacts and form complex synaptic arrangements, TRPM8+ boutons arising from afferents of the same classes of fibers showed a unique synaptic connectivity; simple synapses with one or two dendrites and sparse axoaxonic contacts. These findings suggest that TRPM8-mediated cold is conveyed via a specific subset of C and Aδ afferent neurons and is processed in a unique manner and differently in the TSN and DH.

Illusion caused by vibration of muscle spindles reveals an involvement of muscle spindle inputs in regulating isometric contraction of masseter muscles

Spindle Ia afferents may be differentially involved in voluntary isometric contraction, depending on the pattern of synaptic connections in spindle reflex pathways. We investigated how isometric contraction of masseter muscles is regulated through the activity of their muscle spindles that contain the largest number of intrafusal fibers among skeletal muscle spindles by examining the effects of vibration of muscle spindles on the voluntary isometric contraction. Subjects were instructed to hold the jaw at resting position by counteracting ramp loads applied on lower molar teeth. In response to the increasing-ramp load, the root mean square (RMS) of masseter EMG activity almost linearly increased under no vibration, while displaying a steep linear increase followed by a slower increase under vibration. The regression line of the relationship between the load and RMS was significantly steeper under vibration than under no vibration, suggesting that the subjects overestimated the ramp load and excessively counteracted it as reflected in the emergence of bite pressure. In response to the decreasing-ramp load applied following the increasing one, the RMS hardly decreased under vibration unlike under no vibration, leading to a generation of bite pressure even after the offset of the negative-ramp load until the vibration was ceased. Thus the subjects overestimated the increasing rate of the load while underestimating the decreasing rate of the load, due to the vibration-induced illusion of jaw opening. These observations suggest that spindle Ia/II inputs play crucial roles both in estimating the load and in controlling the isometric contraction of masseter muscles in the jaw-closed position.

Corticofugal direct projections to primary afferent neurons in the trigeminal mesencephalic nucleus of rats

Little is known about projections from the cerebral cortex to the trigeminal mesencephalic nucleus (Vmes) which contains the cell bodies of primary sensory afferents innervating masticatory muscle spindles and periodontal ligaments of the teeth. To address this issue, we employed retrograde (Fluorogold, FG) and anterograde (biotinylated dextranamine, BDA) tracing techniques in the rat. After injections of FG into the Vmes, a large number of neurons were retrogradely labeled in the prefrontal cortex including the medial agranular cortex, anterior cingulate cortex, prelimbic cortex, infralimbic cortex, deep peduncular cortex and insular cortex; the labeling was bilateral, but with an ipsilateral predominance to the injection site. Almost no FG-labeled neurons were found in the somatic sensorimotor cortex. After BDA injections into the prefrontal cortex, anterogradely labeled axon fibers and boutons were distributed bilaterally in a topographic pattern within the Vmes, but with an ipsilateral predominance to the injection site. The rostral Vmes received more preferential projections from the medial agranular cortex, while the deep peduncular cortex and insular cortex projected more preferentially to the caudal Vmes. Several BDA-labeled axonal boutons made close associations (possible synaptic contacts) with the cell bodies of Vmes neurons. The present results have revealed the direct projections from the prefrontal cortex to the primary sensory neurons in the Vmes and their unique features, suggesting that deep sensory inputs conveyed by the Vmes neurons from masticatory muscle spindles and periodontal ligaments are regulated with specific biological significance in terms of the descending control by the cerebral cortex.

A comparison of the postnatal development of muscle-spindle and periodontal-ligament neurons in the mesencephalic trigeminal nucleus of the rat

The trigeminal mesencephalic nucleus (Vmes) is known to include primary afferent neurons of jaw muscle spindles (MS neurons) and periodontal ligament receptors (PL neurons). The aim of this study was to clarify the postnatal development of Vmes neurons by comparing MS neurons with PL neurons using horseradish peroxidase labeling. We measured somal diameter and somal shape of MS and PL neurons in rats from postnatal day (P)7 to P70. No significant changes were seen between postnatal day P7 and P70 in somal diameter or somal shape of MS neurons. Conversely, PL neurons showed a larger somal diameter at P7 than at P14, and in terms of somal profile, multipolar neurons comprised 0% at P7, but 4.8% at P14 and 16.9% at P70. These findings suggest that PL neurons develop with the eruption of teeth, taking into account the fact that tooth eruption occurs from around P14 in rats. Conversely, the lack of postnatal changes in MS neurons is due to the fact that these neurons have been active since the embryonic period, as swallowing starts in utero.

Recruitment of masseter motoneurons by the presumed spindle Ia inputs

The slow-closing phase of the mastication cycle plays a major role in the mastication of foods. However, the neuronal mechanism underlying the slow-closing phase remains unknown. The isometric contraction of jaw-closing muscles is developed through the recruitment of jaw-closing motoneurons during the slow-closing phase. It is well established that motor units are recruited depending on the order of sizes or input resistances (IRs) of motoneurons, which is known as the size principle. Two-pore-domain acid-sensitive K(+) (TASK1/3) channels are recently found to be the molecular correlates of the IR, and also found to be expressed in the masseter motoneurons. Here, we addressed the question whether spindle Ia inputs onto masseter motoneurons can induce the orderly recruitment of motoneurons in slice preparations of the rat brain using voltage-sensitive dye imaging and whole-cell patch-clamp methods. Voltage-sensitive dye imaging revealed the recruitment of many motoneurons in the whole nucleus of masseter in response to repetitive stimulation of the presumed spindle Ia inputs. Dual whole-cell recordings obtained from two adjacent motoneurons revealed the IR-ordered recruitment of motoneurons in response to repetitive stimulation of the presumed spindle Ia inputs. Thus, Ia inputs are likely to play a crucial role in the orderly recruitment of motoneurons of the trigeminal motor nucleus, which would be progressed during the slow-closing phase of the mastication cycle. Possible involvements of TASK channels in the orderly recruitment are discussed.

Spatial distribution patterns of excitatory and inhibitory synapses in the dendritic tree differ between jaw-closing and -opening motoneurons

This paper reviews recent data on the spatial distribution of inhibitory and excitatory synapses on the dendritic tree of jaw-closing (JC) and -opening (JO) motoneurons in the cat, in which a combination of techniques employing intracellular injections of horseradish peroxidase and postembedding immunogold labelling was used. The dendritic tree is divided into three segments: primary and distal dendrites and intermediate dendrites between the two segments. The proportion of inhibitory boutons (immunoreactive for GABA and/or glycine) is slightly higher than proportion of excitatory boutons (immunoreactive for glutamate) in JC motoneurons, but this trend is reversed in JO motoneurons. In the two kinds of motoneuron, boutons immunoreactive to glycine alone are more numerous than boutons double-labelled to GABA and glycine, which, in turn, occur more frequently than boutons immunoreactive to GABA alone. In JC motoneurons, the packing density (number of boutons per 100 microm(2)) of the inhibitory boutons decreases somatofugally, but this trend is not applicable to the excitatory boutons. In contrast, the packing density of the inhibitory and excitatory boutons in JO motoneurons does not significantly differ among the three dendritic compartments, though it is slightly higher for the excitatory than the inhibitory ones on each dendritic segment. These differences have important implications for synaptic integration in JC and JO motoneurons.

NK1 receptor activation by geniohyoid primary afferents modulates parasympathetic postganglionic neuronal excitability in the rat

Previous studies have indicated that the geniohyoid (GH) muscle receives innervation via both the hypoglossal nerve (CNXII) and the ansa cervicalis. Our recent studies revealed that the efferent root that contributes to the ansa cervicalis is a parasympathetic pathway and contains postganglionic cell bodies. Afferent axons from the GH muscle also travel via the ansa cervicalis, and afferent cell bodies are located in spinal ganglia. The present study attempts to locate the central terminations of these afferents. From the peripheral cut end of the ansa cervicalis, we recorded afferent discharges that coincided with inspiration and these were elicited by stretch of the GH muscle. After cutting CNXII proximal to its union with the ansa cervicalis, we applied horseradish peroxidase to the branch of CNXII that innervates the GH muscle. This procedure labeled cells ipsilaterally in the C2 spinal ganglia but not in the brainstem or upper spinal cord. Substance P-reactive terminals in the peripheral CNXII trunk were in apparent contact with vasoactive intestinal peptide-reactive cell bodies. Addition of the NK1 receptor agonist SP(NK1) excited parasympathetic postganglionic neurons and the specific NK1 receptor antagonist GR82334 blocked these effects in vitro. These results suggest that GH primary afferents synapse on parasympathetic postganglionic neurons in the CNXII trunk and that activation of SP(NK1) receptors modulates activity in these neurons.

Distribution of contacts from vestibulospinal axons on the dendrites of splenius motoneurons

Current descriptions of the organization of synapses on the dendritic trees of spinal motoneurons indicate that the inputs are arranged in several patterns: some are widely distributed; some are distributed to proximal dendrites; others are distributed based on the trajectory of the dendrites. However, the principles governing the organization of synapses on spinal motoneurons remain poorly defined. Our goal was to extend the descriptions of the distribution of synapses, identified by their source, on the dendritic trees of spinal motoneurons. We combined anterograde and intracellular staining techniques in cats to determine the distribution of contacts between excitatory axons from the rostral aspect of the descending vestibular nucleus and the dendrites of motoneurons supplying a dorsal neck muscle, splenius. In five of five motoneurons, the contacts were preferentially distributed on dendrites medial to the soma. This qualitative observation was confirmed by using Monte Carlo methods. The results from this analysis showed that the distribution of contacts can be explained not by the overall distribution of the dendritic membrane area but rather by a systematic innervation of the medial regions of the dendritic trees (P < 0.02). Despite this selectivity, there was no additional bias in the distribution of contacts to proximal vs. distal dendrites. By concentrating excitatory synapses in a restricted region of the dendritic tree, the actions of vestibulospinal connections on neck motoneurons may be increased as a result of a greater probability of activating persistent inward currents on the dendrites.

Expression of vesicular glutamate transporter 1 immunoreactivity in peripheral and central endings of trigeminal mesencephalic nucleus neurons in the rat

The major neuronal components of the trigeminal mesencephalic nucleus (Vmes) are primary afferent neurons that convey proprioceptive information from the cranioorofacial regions. In the present study, we examined expression of vesicular glutamate transporters (VGLUTs), VGLUT1 and VGLUT2, in the primary afferent neurons of the Vmes (Vmes neurons) in neonatal and adult rats. VGLUT1 immunoreactivity was detected in the cell bodies of Vmes neurons in neonatal rats younger than 11 days old, but not in older rats. However, in situ hybridization signals for VGLUT1 mRNA were detected in both neonatal and adult rats. No VGLUT2 immunoreactivity was detected in Vmes neurons of neonatal or adult rats. VGLUT1 immunoreactivity was also seen in the peripheral sensory endings on the equatorial regions of intrafusal fibers of muscle spindles in the masseter muscles in both neonatal and adult rats. In adult rats injected with cholera toxin B subunit (CTb) into the masseter nerve, central axon terminals of Vmes neurons were identified on masseter motoneurons within the trigeminal motor nucleus (Vm) by transganglionically and retrogradely transported CTb. VGLUT1-immunopositive axon terminals in close apposition to CTb-labeled Vm motoneurons were also detected by dual-immunofluorescence histochemistry for VGLUT1/CTb. Electron microscopy after dual immunolabeling for VGLUT1/CTb by the VGLUT1/immunoperoxidase and CTb/immunogold-silver methods further revealed synaptic contact of VGLUT1- and CTb-immunopositive axon terminals upon CTb-labeled neuronal profiles within the Vm. These data indicate that VGLUT1 is expressed in both the central axon terminals and the peripheral sensory endings of Vmes neurons, although no VGLUT1 immunoreactivity was detectable in the cell bodies of Vmes neurons in adult rats.

The synaptic microcircuitry associated with primary afferent terminals in the interpolaris and caudalis of trigeminal sensory nuclear complex

Previous ultrastructural studies indicating a higher number of axoaxonic contacts on individual low-threshold mechanoreceptive afferents in the principalis (Vp) than in the oralis (Vo) of cat trigeminal sensory nuclear complex (TSNC) suggest that the synaptic microcircuitry associated with primary afferents manifests unique differences across the sensory nuclei of TSNC. To address this issue, we analyzed synaptic microcircuits associated with fast adapting vibrissa afferent terminals in the interpolaris (Vi) and caudalis (Vc, laminae III/IV) by using intraaxonal injections of horseradish peroxidase (HRP) in cats. Forty-two and 65 HRP-labeled boutons were analyzed in the Vi and Vc, respectively. The labeled boutons contained clear, spherical vesicles. They most frequently formed asymmetric axodendritic synapses and were commonly postsynaptic to unlabeled axon terminals containing pleomorphic vesicles (p-endings) with symmetric junctions. The examination of synaptic contacts over the entire surface of individual boutons indicated that the afferent boutons made contacts with an average of two postsynaptic targets in the Vi and Vc. In contrast, axoaxonic contacts, and labeled boutons participating in synaptic triads, where p-endings contacted both the boutons and their postsynaptic targets, were, on average, higher in the Vi than in the Vc. These results suggest that the output of sensory information conveyed through low-threshold mechanoreceptive afferents is more strongly controlled at the level of the first synapse by presynaptic and postsynaptic mechanisms in the Vi responsible for sensory discriminative functions than in the Vc for sensorimotor reflexive functions.

Neural mechanisms controlling jaw-jerk reflex in the cat

Signal substances of axon terminals presynaptic to jaw spindle Ia afferents and their ultrastructural features were examined using a combination of intra-axonal horseradish peroxidase injection and postembedding immunogold-labeling techniques in cats. A total of 35 axon terminals presynaptic to 22 horseradish peroxidase-labeled Ia boutons were examined. Of the 35 presynaptic axon terminals, 14 (40%) were immunoreactive for both gamma-aminobutyric acid and glycine, 9 (26%) for gamma-aminobutyric acid alone and 9 (26%) for glycine alone. The bouton volume, mitochondrial volume, active zone area, and apposed surface area were larger for Ia boutons than for presynaptic axon terminals, while each of the values is similar among the three types of presynaptic axon terminals. These results suggest that gamma-aminobutyric acid and glycine play an important role for modulating the jaw-jerk reflex presynaptically and that the smaller size of presynaptic axon terminals is important to prevent action potential generation from Ia afferents.

The distribution of inhibitory and excitatory synapses on single, reconstructed jaw-opening motoneurons in the cat

In a previous study, we reported that the distribution of inhibitory input, in contrast to excitatory input, decreased somatofugally along dendrites of cat jaw-closing alpha-motoneurons [J Comp Neurol 414 (1999) 454]. The present study examined the distribution of GABA, glycine, and glutamate immunopositive boutons covering horseradish peroxidase-labeled cat jaw-opening motoneurons. The motoneurons were divided into four compartments: the soma, and primary, intermediate, and distal dendrites. Ninety-seven percent of the total number of studied boutons had immunoreactivity for at least one of the three amino acids. The proportion of boutons immunoreactive for GABA and/or glycine was lower than the proportion of boutons immunoreactive for glutamate. Boutons immunoreactive to glycine alone were more numerous than boutons double-labeled for GABA and glycine, which, in turn, occurred more frequently than boutons immunoreactive to GABA alone. The percentage synaptic covering (proportion of membrane covered by synaptic boutons) of the putatively excitatory (glutamate containing) and putatively inhibitory (GABA and/or glycine containing) boutons decreased somatofugally along the dendrites. Such systematic variations were not seen in the packing density (number of boutons per 100 microm(2)); the packing density showed a distinct drop between the soma and primary dendrites but did not differ significantly among the three dendritic compartments. Overall, the packing density was slightly higher for the putatively excitatory boutons than for the inhibitory ones. When taken together with previous analyses of jaw-closing alpha-motoneurons the present data on jaw-opening alpha-motoneurons indicate that the two types of neuron differ in regard to the nature of synaptic integration in the dendritic tree.

Bilateral projection of functionally characterized trigeminal oralis neurons to trigeminal motoneurons in cats

Intracellular Neurobiotin-injections were used to label functionally identified neurons in the rostro-dorsomedial part of the trigeminal oral nucleus (Vo.r) in the cat. The labeled Vo.r neurons with the mechanoreceptive field in oral tissues innervated bilaterally either jaw-opening motoneurons or jaw-closing motoneurons. This result suggests that Vo.r neurons play an important role in sensory-motor reflexes responsible for coordination of bilaterally symmetrical jaw movements.

Quantitative analysis of the dendritic architectures of single jaw-closing and jaw-opening motoneurons in cats

Little is known about the dendritic architectures of trigeminal motoneurons innervating antagonistic muscles. Thus, the aim of the present study was to provide a quantitative description of jaw-closing (JC) and jaw-opening (JO) alpha motoneurons and to determine geometrical similarities and differences of the dendritic tree between the two. Seven JC alpha motoneurons and four JO alpha motoneurons were intracellularly labeled with horseradish peroxidase (HRP) in the cat and quantitatively analyzed with a computer-assisted three-dimensional system. The dendritic tree of JC alpha motoneurons was confined within the JC motor nucleus, despite locations of the cell body. In contrast, JO alpha motoneurons generated extensive extranuclear dendrites in the reticular formation. The branching pattern of proximal dendritic segments was simpler in the JC than in the JO alpha motoneurons. Despite these differences, the mean values of dendritic parameters examined per neuron were not different between the two kinds of alpha motoneurons, and the stem dendrite diameter was positively correlated with several dendritic parameters in a linear manner. The present study provides new evidence that underlying design principles of the geometry of the dendritic tree are not concerned with the differences in configuration and branching pattern of the dendritic tree of trigeminal alpha motoneurons innervating antagonistic muscles. In addition, we estimated the number of excitatory and inhibitory synapses covering dendrites of single JC alpha motoneurons.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Quantitative analysis of synaptic contacts made between functionally identified oralis neurons and trigeminal motoneurons in cats

A previous study revealed that rostrodorsomedial oralis (Vo.r) neurons synapsing on trigeminal motoneurons use GABA and/or glycine as neurotransmitters. To determine the number and spatial distribution of contacts, injections of biotinamide and horseradish peroxidase were made into a Vo.r neuron and an alpha-motoneuron in the jaw-closing (JC) and jaw-opening (JO) motor nucleus, respectively, in 39 cats. All Vo.r neurons responded to low-threshold mechanical stimulation of the oral tissues. Single Vo.r neurons terminating in the JC nucleus (Vo.r-dl neurons; n = 5) issued, on average, 10 times more boutons than Vo.r neurons terminating in the JO nucleus (Vo.r-vm neurons; n = 5; 4437 vs 445). The Vo.r-dl neuron-JC alpha-motoneuron pairs (n = 4) made contacts on either the soma-dendritic compartment or dendrites, and the Vo.r-vm neuron-JO motoneuron pairs (n = 2) made contacts on dendrites, with a range of two to seven contacts. In five of the six pairs, individual or groups of two to three terminals contacted different dendritic branches of a postsynaptic cell. The Vo.r-dl neurons innervated a greater number of counter-stained motoneuronal somata than did the Vo.r-vm neurons (216 vs 26). Total number of contacts per Vo.r neuron was higher for the Vo.r-dl than Vo.r-vm neurons (786 vs 72). The present study demonstrates that axonal branches of Vo.r neurons are divided into two types with different innervation domains on the postsynaptic neuron and that they are highly divergent. The overall effect exerted by these neurons is predicted to be much greater within the JC than JO motoneuron pool.

Quantitative ultrastructure of slowly adapting lingual afferent terminals in the principal and oral nuclei in the cat

Previous studies provide evidence that a structure/function correlation exists in the cytoarchitectonically different zones of the trigeminal sensory nuclei. To extend this relationship, we examined the ultrastructural features of trigeminal primary afferent neurons in the cat dorsal principal nucleus (Vpd) and the rostrodorsomedial oral nucleus (Vo.r) using intra-axonal labeling with horseradish peroxidase and morphometric analyses. All labeled boutons contained round synaptic vesicles. Eighty-two percent of the boutons in the Vo.r and 99% of the boutons in the Vpd were presynaptic to nonprimary dendrites. The remaining boutons in the Vo.r were presynaptic to somata (8%) or primary dendrites (10%). The average number of postsynaptic profiles per labeled bouton did not differ in the Vpd and Vo.r. Most labeled boutons in the two nuclei were postsynaptic to unlabeled axon terminals with pleomorphic vesicles (p-ending). The number of p-endings per labeled bouton was higher in the Vpd than Vo.r A morphometric analysis indicated that labeled bouton volume and apposed surface area were larger in the Vpd than Vo.r while active zone area and vesicle number did not differ. All these parameters were larger than those of p-endings in each nucleus. In both labeled boutons and p-endings, the parameters were positively correlated with bouton size. These results suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by presynaptic mechanisms in the Vpd than in the Vo.r, while the efficacy of transmission at primary afferent synapses does not differ.

Jaw-muscle spindle afferent pathways to the trigeminal motor nucleus in the rat

Neural pathways conveying proprioceptive feedback from the jaw muscles were studied in rats by combining retrograde and intracellular neuronal labeling. Initially, horseradish peroxidase was iontophoresed unilaterally into the trigeminal motor nucleus (Vmo). Two days later, 1-5 jaw-muscle spindle afferent axons located in the mesencephalic trigeminal nucleus were physiologically identified and intracellularly stained with biotinamide. Stained mesencephalic trigeminal jaw-muscle spindle afferent axon collaterals and boutons were predominantly distributed in the supratrigeminal region (Vsup), Vmo, dorsomedial trigeminal principal sensory nucleus (Vpdm), parvicellular reticular formation (PCRt), alpha division of the parvicellular reticular formation (PCRtA), and dorsomedial portions of the spinal trigeminal subnuclei oralis (Vodm), and interpolaris (Vidm). Numerous neurons retrogradely labeled with horseradish peroxidase from the trigeminal motor nucleus were found bilaterally in the PCRt, PCRtA, Vodm, and Vidm. Retrogradely labeled neurons were also present contralaterally in the Vsup, Vpdm, Vmo, peritrigeminal zone, and bilaterally in the dorsal medullary reticular field. Putative contacts between intracellularly stained mesencephalic trigeminal jaw-muscle spindle afferent boutons and trigeminal premotor neurons retrogradely labeled with horseradish peroxidase were found in the ipsilateral Vodm, PCRtA, and PCRt, as well as the contralateral Vsup, Vmo, Vodm, PCRt, and PCRtA. Thus, multiple disynaptic jaw-muscle spindle afferent-motoneuron circuits exist. These pathways are likely to convey long-latency jaw-muscle stretch reflexes and may contribute to stiffness regulation of the masticatory muscles.

Distribution pattern of inhibitory and excitatory synapses in the dendritic tree of single masseter alpha-motoneurons in the cat

Little is known about the differences in the distributions of inhibitory and excitatory synapses in the dendritic tree of single motoneurons in the brainstem and spinal cord. In this study, the distribution of gamma-aminobutyric acid (GABA)-, glycine-, and glutamate-like immunoreactivity in axon terminals on dendrites of cat masseter alpha-motoneurons, stained intracellularly with horseradish peroxidase, was examined by using postembedding immunogold histochemistry in serial ultrathin sections. The dendritic tree was divided into three segments: primary (Pd) and distal (Dd) dendrites and intermediate (Id) dendrites between the two segments. Quantitative analysis of 175, 279, and 105 boutons synapsing on 13 Pd, 54 Id, and 81 Dd, respectively, was performed. Fifty percent of the total number of studied boutons were immunopositive for GABA and/or glycine and 48% for glutamate. Among the former, 27% showed glycine immunoreactivity only and 14% were immunoreactive to both glycine and GABA. The remainder (9%) showed immunoreactivity for GABA only. As few as 3% of the boutons were immunonegative for the three amino acids. Most boutons immunoreactive to inhibitory amino acid(s) contained a mixture of spherical, oval, and flattened synaptic vesicles. Most boutons immunoreactive to excitatory amino acid contained clear, spherical, synaptic vesicles with a few dense-cored vesicles. When comparisons of the inhibitory and excitatory boutons were made between the three dendritic segments, the proportion of the inhibitory to the excitatory boutons was high in the Pd (60% vs. 37%) but somewhat low in the Id (46% vs. 52%) and Dd (44% vs. 53%). The percentage of synaptic covering and packing density of the inhibitory synaptic boutons decreased in the order Pd, Id, and Dd, but this trend was not applicable to the excitatory boutons. The present study provides possible evidence that the spatial distribution patterns of inhibitory and excitatory synapses are different in the dendritic tree of jaw-closing alpha-motoneurons.

Projections from the lateral vestibular nucleus to the upper cervical spinal cord of the cat: A correlative light and electron microscopic study of axon terminals stained with PHA-L

Vestibulospinal axon collaterals in C1 and C2 were stained following injections of Phaseolus vulgaris leucoagglutinin (PHA-L) into the lateral vestibular nucleus (LVN). The distribution and geometry of collaterals within three regions of the ventral horn were determined at the light microscopic level. These processes were subsequently examined at the electron microscopic level to define the relationship between their ultrastructural characteristics and their geometry and location. All round or elliptical varicosities, whose diameters exceeded the diameter of the adjacent axon shaft by a factor of two, as measured at the light microscopic level, contained synaptic vesicles and contacted dendrites or somata. These varicosities accounted for 82% of labelled axon terminals found at the electron microscopic level. Thus, axon terminals stained with PHA-L can be identified reliably at the light microscopic level, but synaptic density will be slightly underestimated. One-hundred and thirty-eight axon terminals were classified as excitatory or inhibitory on the basis of well-established morphological criteria (e.g., vesicle shape). Placed in the context of previous physiological observations describing the excitatory or inhibitory actions of medial and lateral vestibulospinal tract (MVST and LVST) neurons, our results suggest that projections from the LVN to the ipsilateral ventral horn originate primarily from the LVST. These connections are excitatory. Ipsilateral connections via the MVST are inhibitory and are largely confined to a region near the border of laminae VII and VIII. Most axon terminals in the contralateral ventral horn were inhibitory. This result indicates that the LVN is the source of a specific subset of crossed MVST axons with inputs from the posterior semicircular canal.

Ultrastructural anatomy of physiologically identified jaw-muscle spindle afferent terminations onto retrogradely labeled jaw-elevator motoneurons in the rat

Neuronal microcircuits involving jaw-muscle spindle afferents and jaw-elevator motoneurons were studied via retrograde and intracellular labeling in rats. Initially, trigeminal motoneurons were retrogradely labeled from horseradish peroxidase (HRP) injections into the temporalis and masseter muscles. The intracellular response of jaw-muscle spindle afferent neurons was then characterized during palpation, ramp and hold, and sinusoidal stretching of the jaw-closing muscles. Biotinamide was injected into these neurons, and the tissue was processed for the visualization of HRP and biotinamide. The ultrastructure of 243 intracellularly stained jaw-muscle spindle afferent boutons located within the trigeminal motor nucleus (Vmo) was examined. Eighty-five of these boutons synapsed with motoneurons retrogradely labeled with HRP, and 158 boutons synapsed with unlabeled structures within the Vmo. All spindle afferent boutons contained clear, spherical synaptic vesicles. Although the majority of boutons were S type, a few labeled jaw-muscle spindle afferent boutons possessed a long, narrow cleft, with a subsynaptic cistern comparable to previous descriptions of C-type boutons. Sixty-eight percent of spindle afferent boutons synapsed with large or medium-sized, retrogradely labeled motoneuron dendrites, and 32% synapsed with retrogradely labeled somata. In numerous instances, spindle afferent boutons synapsed with trigeminal motoneuron dendritic or somatic spines. Most of the synapses between spindle afferent boutons and trigeminal motoneuron dendrites were asymmetric, and the greatest percentage of axosomatic synapses between spindle afferents and trigeminal motoneurons were symmetric. Approximately 24% of spindle afferent boutons constituted the intermediate element of a axoaxodendritic or axoaxosomatic assemblage, implying that some jaw-muscle spindle afferent synapses with trigeminal motoneurons are presynaptically modulated.

The trigeminal motonucleus in man

The constitutional elements of the mototrigeminal nucleus in man are described. Apart from the well-known alpha motoneurons, interneurons and gamma motoneurons can be discerned. Cortical projections to the mototrigeminal nucleus in man arise both ipsilateral and contralateral. The contralateral projection is dominant. Terminal cortical input is present on the alpha-motoneurons in man.

Identification of gamma-aminobutyric acid-immunoreactive axon endings associated with mesencephalic periodontal afferent terminals and morphometry of the two types of terminals in the cat supratrigeminal nucleus

A previous study has shown that mesencephalic periodontal afferent terminals receive contacts more frequently from axonal endings containing pleomorphic, synaptic vesicles (P-endings) in the supratrigeminal nucleus (Vsup) than in the trigeminal motor nucleus, suggesting that interneurons in Vsup play an important role in modulating the jaw-closing reflex. The present study was attempted to identify neurotransmitters in P-endings associated with mesencephalic periodontal afferents in cat Vsup through the use of intracellular staining of horseradish peroxidase combined with the postembedding immunogold methods. A morphometric analysis was carried out to compare the ultrastructural features of these two types of terminals. Serial sections of 31 labeled boutons and of their associated 38 P-endings were examined. They were processed for postembedding immunogold labeling with antibodies to the neurotransmitter gamma-aminobutyric acid (GABA). The 38 P-endings presynaptic to periodontal afferents showed GABA-like immunoreactivity, but the afferent terminals were free from the labeling. The morphometric analysis indicated that bouton volume, apposed surface area, total active zone size, and mitochondrial volume were smaller in GABA-immunoreactive P-endings than in periodontal afferents, but the pooled data of the two types of terminals showed that each synaptic parameter was highly correlated in a positive, linear manner with bouton volume. These observations provide evidence that P-endings presynaptic to mesencephalic periodontal afferents contain the neurotransmitter GABA and that their axoaxonic synapses are organized in accordance with the ultrastructural "size principle" proposed by Pierce and Mendell (Pierce and Mendell [1993] J. Neurosci. 13:4748-4763) on Ia-motoneuron synapses.

Ultrastructural observations of synaptic connections of vibrissa afferent terminals in cat principal sensory nucleus and morphometry of related synaptic elements

Previous work suggests that slowly adapting (SA) periodontal afferents have different synaptic arrangements in the principal (Vp) and oral trigeminal nuclei and that the synaptic structure associated with transmitter release may be related directly to bouton size. The present study examined the ultrastructures of SA and fast adapting (FA) vibrissa afferents and their associated unlabeled axonal endings in the cat Vp by using intra-axonal labeling with horseradish peroxidase and a morphometric analysis. All SA and FA afferent boutons contained clear, round, synaptic vesicles. All the FA and most SA boutons were presynaptic to dendrites, but a few SA boutons were axosomatic. Both types of bouton were frequently postsynaptic to unlabeled axonal ending(s) containing pleomorphic, synaptic vesicles (P-ending). The size of labeled boutons was larger in FA than SA afferents, but the size of dendrites postsynaptic to labeled boutons was larger for SA than FA afferents. Large-sized FA and SA boutons made synaptic contacts with small-diameter dendrites. The size of FA and SA boutons was larger than that of their associated P-endings. A morphometric analysis made on the pooled data of SA and FA boutons indicated that apposed surface area, active zone number, total active zone area, vesicle number, and mitochondrial volume were highly correlated in a positive linear manner with labeled bouton volume. These relationships were also applicable to unlabeled P-endings, but the range of each parameter was smaller than that of the labeled boutons. These observations provide evidence that the two functionally distinct types of vibrissa afferent manifest unique differences but share certain structural features in the synaptic organization and that the ultrastructural "size principle" proposed by Pierce and Mendell ([1993] J. Neurosci. 13:4748-4763) for Ia-motoneuron synapses is applicable to the somatosensory system.

Serotonergic axonal contacts on identified cat trigeminal motoneurons and their correlation with medullary raphe nucleus stimulation

The innervation of the trigeminal motor nucleus by serotonergic fibers with cell bodies in the raphe nuclei pallidus and obscurus suggests that activation of this pathway may alter the excitability of trigeminal motoneurons. Thus, we recorded intracellular responses from cat jaw-closing (JC) andjaw-opening (JO) alpha-motoneurons evoked by raphe stimulation and used a combination of intracellular staining of horseradish peroxidase (HRP) and immunohistochemistry at the light and electron microscopic levels to examine the distribution of contacts made by serotonin (5-HT)-immunoreactive boutons on the two motoneurons types. Electrical stimulation applied to the nucleus raphe pallidus-obscurus complex induced a monosynaptic excitatory postsynaptic potential (EPSP) in JC (masseter) alpha-motoneurons and an EPSP with an action potential in JO (mylohyoid) alpha-motoneurons. The EPSP rise-times (time to peak) and half widths were significantly longer in the JC than in the JO motoneurons. The EPSPs were suppressed by systemic administration of methysergide (2 mg/kg). Six JC and seven JO alpha-motoneurons were well stained with HRP. Contacts were seen between 5-HT-immunoreactive boutons and the motoneurons. The JC motoneurons received a significantly larger number of the contacts than did the JO motoneurons. The contacts were distributed widely in the proximal three-fourths of the dendritic tree of JC motoneurons but were distributed on more proximal dendrites in the JO motoneurons. At the electron microscopic level, synaptic contacts made by 5-HT-immunoreactive boutons on motoneurons were identified. The present study demonstrated that JC motoneurons receive stronger 5-HT innervation, and this correlates with the fact that raphe stimulation caused larger EPSPs among these neurons than among JO motoneurons.

Primary- and secondary-like jaw-muscle spindle afferents have characteristic topographic distributions

Single jaw-muscle spindle afferent axons were characterized physiologically and intracellularly stained to determine whether particular physiological types of spindle afferent show distinctive morphologies. Microelectrodes filled with either horseradish peroxidase (HRP) or biotinamide (Neurobiotin) were advanced into the mesencephalic trigeminal nucleus (Vme) in anesthetized rats. Intracellular recordings then were characterized by their response: to palpation of the jaw muscles; when pressure was applied to the teeth and during passive ramp and hold and sinusoidal jaw movement. Seventy-one afferents were characterized physiologically and injected with HRP; an additional 61 afferents were typed and injected with biotinamide. The response of 43 stained neurons was recorded in the presence of suxamethonium. The major projection areas of these afferents were the: trigeminal motor nucleus (Vmo); region dorsal to Vmo; reticular formation, spinal trigeminal nucleus, superior cerebellar peduncle and Vme. One afferent type was modulated strongly during stretching of the jaw-elevator muscles. Based on their high sensitivity during stretching of the jaw muscles and/or their silencing during the release phase of muscle stretch, these afferents were classified as primary-like spindle afferents. These afferents projected most strongly to Vmo. A second type of afferent was modulated only modestly during stretching of the jaw-elevator muscles. These tonic afferents were classified as secondary-like spindle afferents because of their low dynamic sensitivity during ramp muscle stretch and their continued discharge during the release phase of muscle stretch. Secondary-like afferents projected most strongly to the region dorsal to Vmo. Boutons (n = 3,834) from 11 afferents were studied in detail. Secondary-like afferents had statistically larger boutons within Vmo. In both secondary- and primary-like spindle afferents, only a small number of boutons were associated closely with the somata and proximal dendrites of trigeminal motoneurons. In these cases, however, two to five boutons appeared to contact individual motoneurons, implying multiple monosynaptic inputs to a selective subset of jaw-elevator motoneurons. Some "giant" boutons were present dorsal to Vmo and in Vme. These results demonstrate that dynamically sensitive and nondynamically sensitive jaw-elevator muscle spindle afferents project preferentially to different regions. Primary-like spindle afferents are capable of providing feedback related to the dynamic phases of muscle stretch and project most heavily to Vmo. Secondary-like spindle afferents can transmit a feedback signal associated with muscle length and project most strongly to the supratrigeminal region. Both types of afferent have projections caudal to Vmo that may serve longer latency jaw-muscle stretch reflexes and/or the projection of proprioceptive information to the thalamus and cerebellum.

Electron microscopic observation of synaptic connections of jaw-muscle spindle and periodontal afferent terminals in the trigeminal motor and supratrigeminal nuclei in the cat

Previous studies indicate that the trigeminal motor nucleus (Vmo) and supratrigeminal nucleus (Vsup) receive direct projections from muscle spindle (MS) and periodontal ligament (PL) afferents. The aim of the present study is to examine the ultrastructural characteristics of the two kinds of afferent in both nuclei using the intracellular horseradish peroxidase (HRP) injection technique in the cat. Our observations are based on complete or near-complete reconstructions of 288 MS (six fibers) and 69 PL (eight fibers) afferent boutons in Vmo, and of 93 MS (four fibers) and 188 PL (four fibers) afferent boutons in Vsup. All the labeled boutons contained spherical synaptic vesicles and were presynaptic to neuronal elements, and some were postsynaptic to axon terminals containing pleomorphic, synaptic vesicles (P-endings). In Vmo neuropil, MS afferent boutons were distributed widely from soma to distal dendrites, but PL afferent boutons predominated on distal dendrites. Most MS afferent boutons (87%) formed synaptic specialization(s) with one postsynaptic target while some (13%) contacting two or three dendritic profiles; PL afferents had a higher number of boutons (43%) contacting two or more dendritic profiles. A small but significant number of MS afferent boutons (12%) received contacts from P-endings, but PL afferent boutons (36%) received three times as many contacts from P-endings as MS afferents. In Vsup neuropil, most MS (72%) and PL (87%) afferent boutons formed two contacts presynaptic to one dendrite and postsynaptic to one P-ending, and their participation in synaptic triads was much more frequent than in Vmo neuropil. The present study indicates that MS and PL afferent terminals have a distinct characteristic in synaptic arrangements in Vmo and Vsup and provides evidence that the synaptic organization of primary afferents differs between the neuropils containing motoneurons and their interneurons.